Methods and systems for estimating transmit attenuation for a magnetic resonance imaging scan

ABSTRACT

Various methods and systems are provided for correcting transmit attenuation of an amplifier of a transmit radio frequency (RF) coil for use in a magnetic resonance imaging (MRI) system. In one example, a method includes setting a reference value of transmit attenuation for an amplifier of a transmit radio frequency (RF) coil, acquiring a two-dimensional B 1  field map with the transmit attenuation set at the reference value, determining a mean flip angle from the B 1  field map, determining a transmit attenuation correction value based on a prescribed flip angle and the mean flip angle, correcting the reference value of transmit attenuation with the transmit attenuation correction value to obtain a final value of transmit attenuation, and performing an MRI scan with the transmit attenuation set at the value.

FIELD

Embodiments of the subject matter disclosed herein relate to magneticresonance imaging, and more particularly, to estimating transmitattenuation (or transmit gain) for a magnetic resonance imaging scan.

BACKGROUND

Magnetic resonance imaging (MRI) is a medical imaging modality that cancreate images of the inside of a human body without using x-rays orother ionizing radiation. MRI uses a powerful magnet to create a strong,uniform, static magnetic field B₀. When the human body, or part of thehuman body, is placed in the magnetic field B₀, the nuclear spinsassociated with the hydrogen nuclei in tissue water become polarized,wherein the magnetic moments associated with these spins becomepreferentially aligned along the direction of the magnetic field B₀,resulting in a small net tissue magnetization along that axis. MRIsystems also include gradient coils that produce smaller amplitude,spatially-varying magnetic fields with orthogonal axes to spatiallyencode the magnetic resonance (MR) signal by creating a signatureresonance frequency at each location in the body. The hydrogen nucleiare excited by a radio frequency signal at or near the resonancefrequency of the hydrogen nuclei, which add energy to the nuclear spinsystem. As the nuclear spins relax back to their rest energy state, theyrelease the absorbed energy in the form of an RF signal. This RF signal(or MR signal) is detected by one or more RF coil arrays and istransformed into the image using reconstruction algorithms.

The radio frequency signal that is transmitted to add energy to thenuclear spin system (referred to as an RF pulse) generates ahigh-frequency magnetic field B₁. The magnitude of the B₁ field may bevaried in order to generate a desired flip angle, which is the amount ofrotation the net magnetization experiences during the application of theRF pulse. To vary the magnitude of the B₁ field, the power output of thetransmit RF coil(s) may be varied by adjusting the transmit attenuation(or transmit gain) of the RF coil amplifier.

BRIEF DESCRIPTION

In one embodiment, a method for a magnetic resonance imaging (MRI)system includes setting a reference value of transmit attenuation for anamplifier of a transmit radio frequency (RF) coil, acquiring atwo-dimensional B₁ field map with the transmit attenuation set at thereference value, determining a mean flip angle from the B₁ field map,determining a transmit attenuation correction value based on aprescribed flip angle and the mean flip angle, correcting the referencevalue of transmit attenuation with the transmit attenuation correctionvalue to obtain a final value of transmit attenuation, and performing anMRI scan with the transmit attenuation set at the final value.

It should be understood that the brief description above is provided tointroduce in simplified form a selection of concepts that are furtherdescribed in the detailed description. It is not meant to identify keyor essential features of the claimed subject matter, the scope of whichis defined uniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from reading thefollowing description of non-limiting embodiments, with reference to theattached drawings, wherein below:

FIG. 1 is a block diagram of an example MRI system.

FIG. 2 illustrates a process for estimating transmit attenuation with amagnitude-based method.

FIG. 3 illustrates a process for estimating transmit attenuation with aphase-based method.

FIGS. 4A and 4B show vector additions performed during a projection ofthe phase-based method of FIG. 3.

FIG. 5 is a flow chart illustrating a method for correcting transmitattenuation to be applied during an MRI scan.

FIG. 6A is an example phase shift map and FIG. 6B is an example B₁ fieldmap.

FIGS. 7A and 7B show example MR images of a spine.

DETAILED DESCRIPTION

The following description relates to various embodiments for adjustingtransmit attenuation (or transmit gain) in an MRI system. During an MRIscan, hydrogen nuclei are excited by a radio frequency (RF) signal at ornear the resonance frequency of the hydrogen nuclei to add energy to thenuclear spin system. This RF signal is emitted by a transmit RF coil ofthe MRI system, which thereby generates a high-frequency magnetic fieldB₁. The transmit RF signal may be controlled in order to achieve atarget flip angle. The flip angle refers to the amount of rotation thenet magnetization experiences during the application of the RF pulse.The flip angle may be controlled by varying the magnitude of the B₁field, which is a function of the output voltage and current of the RFamplifier of the transmit RF coil. Thus, the transmit attenuation refersto the amount that the maximum output of the RF amplifier is reduced, orattenuated, in order to output an RF signal that achieves the targetflip angle. The RF amplifier output is attenuated by a transmitattenuator (e.g., which may be an interposed circuit) that controls thegain of the RF amplifier so that the commanded current and voltage aresent to the RF coil to generate the desired B₁ field. In someembodiments, “transmit gain” is used to describe the gain of the RFamplifier, which is complementary to the transmit attenuation. Although“transmit attenuation” is used in most part of this disclosure, itshould be understood that the methods disclosed herein apply to“transmit gain” as well.

The value of transmit attenuation of the RF coil (and thus the B₁ fieldgenerated by the RF coil at a given power of the RF coil) may bedetermined during a pre-scan routine prior to initiation of a fullimaging scan, as the value of transmit attenuation may vary from patientto patient and anatomy to anatomy due to variation in size, shape, andcomposition (e.g., fat content, water content, etc.) among patients anddifferent anatomy, which create different electromagnetic loads on theRF coils. The value of transmit attenuation to achieve a desired flipangle may then be set for the full imaging (e.g., the imaging scan wherehigh resolution MR data is obtained to reconstruct one or more images ofthe imaging subject) accordingly. Prior methods for determining thevalue of transmit attenuation include a magnitude-based calculation thatcalculates the area under a curve resulting from the projection of MRsignal magnitude (also referred to as strength) of a 2D slice of theimaging subject acquired during the pre-scan to one dimension. However,the magnitude-based method is relatively time-consuming (e.g., 20-30seconds) to complete because multiple values of transmit attenuationneed to be tested. A phase-based method for determining the value oftransmit attenuation is based on the Bloch-Siegert shift and includes aone-dimensional phase projection where the amount of phase shiftresulting from the application of an off-resonant pulse may be measured(as the amount of the phase shift may depend on the RF pulse magnitude).While this phase-based method is relatively fast, the accuracy of thephase shift estimation relies on a homogenous MR signal magnitude. Thus,if the MR signal strength is inhomogeneous, the method may overestimate(or underestimate) the transmit attenuation, resulting in over or underflip and causing image artifacts.

Thus, according to embodiments disclosed herein, a reference value oftransmit attenuation which is predetermined based on RF coil parametersand anatomy to be scanned may be corrected according to a measured meanflip angle (also called actual flip angle). In particular, the referencevalue of transmit attenuation is set according to a desired (e.g.,prescribed or predetermined) flip angle. A B₁ field map may be acquiredwith the RF amplifier set at the reference value during a pre-scan. Theactual mean flip angle may then be derived from the B₁ field map and atransmit attenuation error (also referred to as a correction value orchange in transmit attenuation) may be calculated based on thepredetermined and actual mean flip angles. The reference value oftransmit attenuation may be corrected by the transmit attenuation error,and a full imaging scan may be carried out with the corrected value oftransmit attenuation. By using a 2D B₁ field map instead of a 1Dprojection, any B₁ variation at multiple locations is preserved, therebyincreasing the accuracy of the transmit attenuation determination.

FIG. 1 illustrates a magnetic resonance imaging (MRI) apparatus 10 thatincludes a magnetostatic field magnet unit 12, a gradient coil unit 13,one or more local RF coil arrays (210, 220, and 230), an RF body coilunit 15, a transmit/receive (T/R) switch 20, an RF port interface 21, anRF driver unit 22, a gradient coil driver unit 23, a data acquisitionunit 24, a controller unit 25, a patient bed 26, a data processing unit31, an operating console unit 32, and a display unit 33. The MRIapparatus 10 transmits electromagnetic pulse signals to a subject 16placed in an imaging space 18 with a magnetostatic field formed toperform a scan for obtaining magnetic resonance (MR) signals from thesubject 16 to reconstruct an image of the slice of the subject 16 basedon the MR signals thus obtained by the scan.

The magnetostatic field magnet unit 12 includes, for example, typicallyan annular superconducting magnet, which is mounted within a toroidalvacuum vessel. The magnet defines a cylindrical space surrounding thesubject 16, and generates a constant primary magnetostatic field B₀.

The MRI apparatus 10 also includes a gradient coil unit 13 that forms agradient magnetic field in the imaging space 18 so as to provide themagnetic resonance signals received by the RF coil arrays withthree-dimensional positional information. The gradient coil unit 13includes three gradient coil systems, each of which generates a gradientmagnetic field which inclines into one of three spatial axesperpendicular to each other, and generates a gradient field in each of afrequency encoding direction, a phase encoding direction, and a sliceselection direction in accordance with the imaging condition. Morespecifically, the gradient coil unit 13 applies a gradient field in theslice selection direction (or scan direction) of the subject 16, toselect the slice; and the RF body coil unit 15 or the local RF coilarrays may transmit an RF pulse to a selected slice of the subject 16.The gradient coil unit 13 also applies a gradient field in the phaseencoding direction of the subject 16 to phase encode the magneticresonance signals from the slice excited by the RF pulse. The gradientcoil unit 13 then applies a gradient field in the frequency encodingdirection of the subject 16 to frequency encode the magnetic resonancesignals from the slice excited by the RF pulse.

Three local RF coil arrays 210, 220, and 230 are shown herein. The localRF coil arrays are disposed, for example, to enclose the region to beimaged of the subject 16. In the static magnetic field space or imagingspace 18 where a static magnetic field B₀ is formed by the magnetostaticfield magnet unit 12, the local RF coil arrays may transmit, based on acontrol signal from the controller unit 25, an RF pulse that is anelectromagnet wave to the subject 16 and thereby generates ahigh-frequency magnetic field B₁. This excites a spin of protons in theslice to be imaged of the subject 16. The local RF coil arrays receive,as a MR signal, the electromagnetic wave generated when the proton spinreturns into alignment with the initial magnetization vector. In oneembodiment, the local RF coil may transmit and receive an RF pulse usingthe same local RF coil. In another embodiment, the local RF coil may beused for only receiving the MR signals, but not transmitting the RFpulse.

The RF body coil unit 15 is disposed, for example, to enclose theimaging space 18, and produces RF magnetic field pulses B₁ orthogonal tothe main magnetic field B₀ produced by the magnetostatic field magnetunit 12 within the imaging space 18 to excite the nuclei. In contrast tothe local RF coil arrays (such as local RF coil arrays 210 and 220),which may be easily disconnected from the MR apparatus 10 and replacedwith another local RF coil, the RF body coil unit 15 is fixedly attachedand connected to the MR apparatus 10. Furthermore, whereas coil arrayscan transmit to or receive signals from only a localized region of thesubject 16, the RF body coil unit 15 generally has a larger coveragearea and can be used to transmit or receive signals to the whole body ofthe subject 16. Using receive-only RF coil arrays and transmit bodycoils provides a uniform RF excitation and good image uniformity at theexpense of high RF power deposited in the subject. For atransmit-receive RF coil array, the coil array provides the RFexcitation to the region of interest and receives the MR signal, therebydecreasing the RF power deposited in the subject. It should beappreciated that the particular use of the local RF coil arrays and/orthe RF body coil unit 15 depends on the imaging application.

The T/R switch 20 can selectively electrically connect the RF body coilunit 15 to the data acquisition unit 24 when operating in receive mode,and to the RF driver unit 22 when operating in transmit mode. Similarly,the T/R switch 20 can selectively electrically connect one or more ofthe local RF coil arrays to the data acquisition unit 24 when the localRF coil arrays operate in receive mode, and to the RF driver unit 22when operating in transmit mode. When the local RF coil arrays and theRF body coil unit 15 are both used in a single scan, for example if thelocal RF coil arrays are configured to receive MR signals and the RFbody coil unit 15 is configured to transmit RF signals, then the T/Rswitch 20 may direct control signals from the RF driver unit 22 to theRF body coil unit 15 while directing received MR signals from the localRF coil arrays to the data acquisition unit 24. The RF body coil unit 15may be configured to operate in a transmit-only mode, a receive-onlymode, or a transmit-receive mode. The local RF coil arrays may beconfigured to operate in a transmit-receive mode or a receive-only mode.

The RF driver unit 22 includes a gate modulator (not shown), an RF poweramplifier 122, and an RF oscillator (not shown) that are used to drivethe RF coil arrays and form a high-frequency magnetic field in theimaging space 18. The RF driver unit 22 modulates, based on a controlsignal from the controller unit 25 and using the gate modulator, the RFsignal received from the RF oscillator into a signal of predeterminedtiming having a predetermined envelope. The RF signal modulated by thegate modulator is amplified by the RF power amplifier and then output tothe RF coil arrays. As will be described in more detail below, the RFamplifier 122 may be attenuated (e.g., reduce the output voltage andcurrent to the RF coils) by a determined amount in order to create adesired B₁ magnetic field.

The gradient coil driver unit 23 drives the gradient coil unit 13 basedon a control signal from the controller unit 25 and thereby generates agradient magnetic field in the imaging space 18. The gradient coildriver unit 23 includes three systems of driver circuits (not shown)corresponding to the three gradient coil systems included in thegradient coil unit 13.

The data acquisition unit 24 includes a preamplifier (not shown), aphase detector (not shown), and an analog/digital converter (not shown)used to acquire the MR signals received by the local RF coil arrays. Inthe data acquisition unit 24, the phase detector phase detects, usingthe output from the RF oscillator of the RF driver unit 22 as areference signal, the MR signals received from the RF coil arrays andamplified by the preamplifier, and outputs the phase-detected analogmagnetic resonance signals to the analog/digital converter forconversion into digital signals. The digital signals thus obtained areoutput to the data processing unit 31.

The MRI apparatus 10 includes a table 26 for placing the subject 16thereon. The subject 16 may be moved inside and outside the imagingspace 18 by moving the table 26 based on control signals from thecontroller unit 25. One or more of the RF coil arrays may be coupled tothe table 26 and moved together with the table.

The controller unit 25 includes a computer and a recording medium onwhich a program to be executed by the computer is recorded, in someembodiments. The program when executed by the computer causes variousparts of the apparatus to carry out operations corresponding topredetermined scanning protocols or settings. The recording medium maycomprise, for example, a ROM, flexible disk, hard disk, optical disk,magneto-optical disk, CD-ROM, or non-volatile memory card. Thecontroller unit 25 is connected to the operating console unit 32 andprocesses the operation signals input to the operating console unit 32and furthermore controls the table 26, RF driver unit 22, gradient coildriver unit 23, and data acquisition unit 24 by outputting controlsignals to them. The controller unit 25 also controls, to obtain adesired image, the data processing unit 31 and the display unit 33 basedon operation signals received from the operating console unit 32.

The operating console unit 32 includes user input devices such as akeyboard and a mouse. The operating console unit 32 is used by anoperator, for example, to input such data as an imaging protocol and toset a region where an imaging sequence is to be executed. The data aboutthe imaging protocol and the imaging sequence execution region areoutput to the controller unit 25.

The data processing unit 31 includes a computer and a recording mediumon which a program to be executed by the computer to performpredetermined data processing is recorded. The data processing unit 31is connected to the controller unit 25 and performs data processingbased on control signals received from the controller unit 25. The dataprocessing unit 31 is also connected to the data acquisition unit 24 andgenerates spectrum data by applying various image processing operationsto the magnetic resonance signals output from the data acquisition unit24.

The display unit 33 includes a display device and displays an image onthe display screen of the display device based on control signalsreceived from the controller unit 25. The display unit 33 displays, forexample, an image regarding an input item about which the operatorinputs operation data from the operating console unit 32. The displayunit 33 also displays a slice image of the subject 16 generated by thedata processing unit 31.

As explained previously, the value of transmit attenuation may bedetermined prior to conducting an imaging scan in order to account forvariable electromagnetic loading different patients and differentanatomies may provide. A first method for determining the value oftransmit attenuation includes a magnitude-based calculation. An examplemagnitude-based calculation process 200 is shown in FIG. 2. Themagnitude-based calculation process 200 includes acquiring a magnitudecurve 204 that extends along one dimension (e.g., the medial-lateralaxis) of a two-dimensional slice 202 of the imaging subject. Slice 202is a cross-sectional slice (e.g., taken along a transverse plane).Magnitude curve 204 represents the projection of MR signal magnitudealong the medial-lateral axis. The area under the curve 204 iscalculated to determine the magnitude of MR signal across the 2D slice202. The magnitude of MR signal is obtained for multiple values oftransmit attenuation before the ideal magnitude is achieved. As such,the magnitude-based method is relatively time-consuming (e.g., taking20-30 seconds to complete), and thus may be less desirable fortime-sensitive imaging protocols.

A second method for determining the value of transmit attenuationincludes a phase-based calculation. An example phase-based calculationprocess 300 is shown in FIG. 3. The phase-based calculation process 300includes acquiring phase curve(s) 304 along one dimension (e.g., themedial-lateral axis) of a 2D slice 302. In FIG. 3, two curves are shown,each corresponding to a channel of the transmit RF coil. Curve(s) 304represents the projection of phase shift along the medial-lateral axisresulting from the application of an off-resonant pulse. The amount ofthe phase shift may depend on the RF pulse magnitude. Based on the phaseshift obtained from curve(s) 304, the desired value of the transmitattenuation is determined. While this phase-based method is relativelyfast, the accuracy of the phase shift estimation using one-dimensionalprojection relies on a homogenous MR signal magnitude. For example,region A of slice 302 may have a different MR signal strength thanregion B. When the projection is performed, the MR signal magnitudeinhomogeneity may result in the method overestimating (orunderestimating) the transmit attenuation.

FIG. 4A shows a first vector addition 400 of regions A and B, where theMR signal magnitudes at regions A and B are homogenous, and FIG. 4Bshows a second vector addition 410 of regions A and B where regions Aand B are not homogenous (such as the example shown in slice 302). If|Vector_(A)|=|Vector_(B)|, Phase_(AB)=(Phase_(A)+Phase_(B))/2, and theprojection reflects the average phase shift in this condition. If|Vector_(A)|≠|Vector_(B)|, Phase_(AB)≠(Phase_(A)+Phase_(B))/2, and theprojection reflects the phase of the averaged vectors (i.e., the solidline in FIG. 4B) but not the average phase shift (i.e., the dashed linein FIG. 4B) in this condition. Thus, the phase shift estimation is notaccurate for inhomogeneous MR signal. As will be explained in moredetail below, a more accurate estimation of transmit attenuation may beprovided by avoiding the phase projection and calculating a mean flipangle directly from the B₁ field map.

FIG. 5 is a flow chart illustrating a method 500 for correcting transmitattenuation of an amplifier driving (e.g., providing a desired outputcurrent and voltage to) a transmit RF coil, such as amplifier 122 of RFdriver unit 22 which drives RF body coil unit 15. Method 500 may beexecuted by controller unit 25 of FIG. 1 according to instructionsstored in non-transitory memory.

At 502, patient information and/or a scanning protocol is received. Forexample, an operator of the MRI system may input a patient identifier,such as a code or the patient's name, and/or the operator may inputselect information about the patient (e.g., date of birth, age, gender,body weight). Further, the operator may select a predefined scanningprotocol from a menu or the operator may input various scanningparameters to set the scanning protocol. The scanning protocol mayindicate the anatomy to be scanned, diagnostic goal of the scan, and/orother parameters that the MRI system may use to identify table position,which receive RF coil arrays are to be used during the scan (e.g., ahead and neck RF coil array, posterior RF coil array, and/or anterior RFcoil array), and other scanning parameters. In particular, the operatormay select the protocol based on the anatomy to be scanned.

Receiving the patient information and/or scanning protocol may include,as indicated at 503, determining a reference value of transmitattenuation (TA) (or transmit gain) based on the scanning protocoland/or patient information. The reference value of TA may define theattenuation to be applied to the amplifier to reduce (e.g., attenuate)the output voltage and current used to drive the transmit RF coil(s) inorder to generate a desired B₁ magnetic field. By adjusting the outputvoltage and current supplied to the transmit RF coil(s), the strength ofthe B₁ magnetic field may be adjusted to the desired strength. Thedesired B₁ magnetic field (and hence reference value of TA) may bedetermined based on the anatomy to be scanned as determined from thescanning protocol and/or patient information, and may be further basedon the configuration of the transmit RF coil(s) (e.g., size, number,etc.). The reference value of TA may be obtained from, for example, alook-up table stored in memory of the controller unit, where the look-uptable indexes reference TA to anatomy to be scanned or scanningprotocol, for example.

At 504, method 500 includes performing a pre-scan. The pre-scan mayinclude activating the MRI system in one or more predetermined sequencesin order to obtain data to calibrate various aspects of the MRI systembefore initiating an imaging scan. The aspects of the MRI system thatmay be calibrated during the pre-scan may include quick higher-ordershimming, coil tuning/matching, center frequency calibration,transmitter gain adjustment, receiver gain adjustment, and dummy cyclestimulation. In some examples, the pre-scan may also include alow-resolution scan of the field of view (FOV) of the MRI system wherethe acquired image is used to reconstruct an MR image of the FOV toconfirm that the desired anatomy is within the FOV. In some examples,this information may be obtained during a separate localizer scanperformed before the pre-scan.

Performing the pre-scan includes acquiring a two-dimensional B₁ fieldmap with transmit attenuation set at the reference value, as indicatedat 505. As mentioned above, the pre-scan may be performed in order toobtain information that is usable to calibrate various aspects of theMRI system, including calibrating the transmit attenuation (or thetransmit gain). To calibrate the transmit attenuation (or the transmitgain), a 2D B₁ field map is acquired. The 2D B₁ field map represents themeasured strength of the B₁ field in the FOV of the MRI system, for eachpixel of a desired plane in imaging space. To acquire the B₁ field map,the MRI system may be operated according to a pulse sequence to causethe nuclei in the bore of the imaging system to undergo a Bloch-Siegertshift. In one example, the pulse sequence includes applying a firstresonating RF pulse to the plurality of nuclei. A resonating RF pulse isan RF pulse tuned to a resonant frequency of a plurality of nucleisubjected to a magnetic field. As such, the application of a resonatingRF pulse places the nuclei in an excited state. The pulse sequence mayfurther include, after application of the first resonating RF pulse,applying a first off-resonance RF pulse to the plurality of excitednuclei. An off-resonance RF pulse is an RF pulse tuned such that thatapplication of the off-resonance RF pulse to a plurality of nuclei doesnot place the plurality of nuclei in an excited state. For example, anoff-resonance RF pulse is an RF pulse having a particular shape orfrequency such that the application thereof to a plurality of nucleisubjected to a magnetic field will not be excited, or will be excited toa minimal extent. The application of this first off-resonance RF pulseoccurs while the plurality of nuclei are already in an excited state.The application of the first off-resonance RF pulse causes the resonancefrequency of the plurality of excited nuclei to shift. Such a shift isoften referred to as a Bloch-Siegert shift. The magnitude of such ashift is dependent on the B₁ field applied to the plurality of excitednuclei.

After application of the off-resonance RF pulse, a first signal isacquired with the receive RF coils. One or more second resonating RFpulses are then applied, without application of any off-resonance RFpulses. A second signal is acquired with the receive RF coils afterapplication of the second resonating RF pulse. The phase shift may bedetermined based on a phase difference of the first and second signals,and then the B₁ field strength may be determined based on the phaseshift.

The 2D B₁ field map may be acquired at any desired location and it canbe acquired from single or multiple slices of the FOV. In one example,the 2D B₁ field map may be acquired at a center-most location of thebore of the MRI system. In another example, the 2D B₁ field map may beacquired at a center slice location of the imaging FOV, or othersuitable location.

The 2D B₁ field map pulse sequence can be acquired in any suitableplane. The sequence gradients are modified in the X, Y and Z directionsto acquire the field map in the desired plane/orientation.

In some examples, method 500 includes optionally, at 506, applying amask to the B₁ field map in order to remove noise. For example, the maskmay be circular mask sized, shaped, and positioned to correspond to thebore of the MRI system. In this way, any B₁ field measured outside thebore (which may correspond to noise) may be removed prior to furtherprocessing. In some embodiments, no mask is applied.

FIG. 6A illustrates an example MR signal phase shift map 600 where eachpixel represents the amount of phase shift. The map of MR signal phaseshift may be acquired by using a Bloch-Siegert shift. For example, themap may be acquired by applying a pulse sequence that induces aBloch-Siegert shift, as described above. A mask 602 may be applied,where the mask is shaped to match the size and shape of the bore. FIG.6B illustrates a corresponding B₁ field map 610 where each pixelrepresents the B₁ field strength. The B₁ field strength may be derivedfrom the MR phase shift according to known relationship between the B₁field strength and MR phase shift. With the mask 602 being applied, theB₁ field strength values lying outside the mask are removed prior tofurther processing of the B₁ field map.

At 508, a mean flip angle (R) is determined from the B₁ field map. TheB₁ field map includes a B₁ field strength value for each pixel inimaging space of the desired slice (e.g., a center slice located in acenter of the bore), as described above. For each pixel, a correspondingflip angle may be calculated based on the B₁ field strength. Forexample, the flip angle may be calculated according to the followingequation:φ=γB ₁ T

Where φ is the flip angle, γ is the gyromagnetic ratio, and T is theduration of the RF pulse. The mean flip angle (R) may then be calculatedby averaging all the calculated flip angles. In some embodiments, a meanB₁ field strength value may be determined by averaging values of the B₁field strength for each pixel, and then calculating the mean flip anglefrom the mean B₁ field value.

At 510, a delta in transmit attenuation (ΔTA), otherwise referred to asthe TA error, is determined based on a prescribed flip angle (R0) andmean flip angle (R). The prescribed flip angle (R0) may be the flipangle that is desired for the upcoming imaging scan based on the anatomybeing scanned and/or scanning protocol (e.g., diagnostic goal of thescan). The flip angle depends on the B₁ field strength. In someembodiments, the reference value of TA discussed above may be selectedin order to produce the prescribed flip angle. However, because the B₁field depends on the loading of the transmit RF coils, which varies frompatient to patient, the reference value of TA may not necessarilygenerate the prescribed flip angle. Thus, the actual flip angle derivedfrom the B₁ field map is used to adjust the transmit attenuation inorder to achieve the prescribed flip angle. In one example, ΔTA may bedetermined based on the equation:ΔTA=−200*log 10(R/R0)

The above equation converts the ratio of the actual flip angle to theexpected flip angle into decibels (specifically, tenths of decibels),but other equations are possible depending on how the transmitattenuation is represented. As an example, using the above equation, ifthe prescribed flip angle is 20° and the measured mean flip angle is24.47°, the ΔTA will be approximately −18 (−1.8 dB).

At 512, method 500 includes determining a final value of TA based on thereference value of TA and ΔTA. For example, the final TA may becalculated by correcting the reference TA with ΔTA. In the aboveexample, if the reference value of TA was 191 (19.1 dB), the final valueof TA may be 173 (17.1 dB).

At 514, an imaging scan is performed using the corrected, final TA. Forexample, the gain of the amplifier may be adjusted to the final value ofTA in order to generate a B₁ field strength that results in theprescribed flip angle being reached. During the imaging scan, MR signalsare obtained (e.g., by the receive RF coils) in response to RF pulsesapplied by the transmit RF coils, and one or more images may bereconstructed from the obtained MR signals.

In this way, one or more transmit RF pulses are applied to an imagingsubject from a transmit RF coil during an imaging scan in order togenerate MR signals that may be received by one or more receive RFcoils. The received MR signals are then used to generate one or moreimages of the imaging subject. The transmit RF coil may be driven at apower output that is determined based on a mean flip angle measured froma two-dimensional B₁ field map. For example, the transmit RF coil may beconfigured with a maximum power output and the transmit RF coil may bedriven with a power output that is less than the maximum power output.The power output at which the RF coil is actually driven is determinedfrom the B₁ field map as described above, whereby the mean flip angle ismeasured from the B₁ field map and a ratio of the mean flip angle to aprescribed flip angle is used to correct a reference power output. Thereference power output may be the power output used to obtain the B₁field map and may be selected to achieve the prescribed flip angle.

FIGS. 7A and 7B show example images reconstructed from MR signalsobtained with a first transmit attenuation calculated with thephase-based method (image 700 of FIG. 7A) and from MR signals obtainedwith a second transmit attenuation calculated according to the methoddescribed above with respect to FIG. 5 (image 710 of FIG. 7B). Image 700includes shading/banding artifacts in the region of the lower spine, ashighlighted by the arrows. In contrast, image 710 does not include thebanding artifacts and the overall brightness and contrast of the spineis increased.

The technical effect of determining a transmit attenuation for driving atransmit RF coil based on a two-dimensional B₁ field map is that thetransmit attenuation determination may be less prone to error resultingfrom B₁ field inhomogeneity. Another technical effect of determining thetransmit attenuation based on the two-dimensional B₁ field map isreduction in banding artifacts in resultant MR images.

As used herein, an element or step recited in the singular and proceededwith the word “a” or “an” should be understood as not excluding pluralof said elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of the present invention arenot intended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features. Moreover, unlessexplicitly stated to the contrary, embodiments “comprising,”“including,” or “having” an element or a plurality of elements having aparticular property may include additional such elements not having thatproperty. The terms “including” and “in which” are used as theplain-language equivalents of the respective terms “comprising” and“wherein.” Moreover, the terms “first,” “second,” and “third,” etc. areused merely as labels, and are not intended to impose numericalrequirements or a particular positional order on their objects.

This written description uses examples to disclose the invention,including the best mode, and also to enable a person of ordinary skillin the relevant art to practice the invention, including making andusing any devices or systems and performing any incorporated methods.The patentable scope of the invention is defined by the claims, and mayinclude other examples that occur to those of ordinary skill in the art.Such other examples are intended to be within the scope of the claims ifthey have structural elements that do not differ from the literallanguage of the claims, or if they include equivalent structuralelements with insubstantial differences from the literal languages ofthe claims.

The invention claimed is:
 1. A method for a magnetic resonance imaging(MRI) system, comprising: setting a reference value of transmitattenuation for an amplifier of a transmit radio frequency (RF) coil;acquiring a two-dimensional B₁ field map with the transmit attenuationset at the reference value; determining a mean flip angle from the B₁field map by averaging values from the B₁ field map; determining atransmit attenuation correction value based on a ratio of the mean flipangle to a prescribed flip angle; correcting the reference value oftransmit attenuation with the transmit attenuation correction value toobtain a final value of transmit attenuation; and performing an MRI scanwith the transmit attenuation set at the final value.
 2. The method ofclaim 1, wherein acquiring the two-dimensional B₁ field map comprises:acquiring a two-dimensional map of magnetic resonance (MR) signal phaseshift during a pre-scan; and deriving the two-dimensional B₁ field mapby converting the MR signal phase shift into corresponding B₁ fieldstrength.
 3. The method of claim 2, wherein the map of MR signal phaseshift is acquired by using Bloch-Siegert shift.
 4. The method of claim1, wherein the reference value of transmit attenuation is set based onone or more anatomical features of an imaging subject to be scanned. 5.The method of claim 1, wherein determining the mean flip angle from theB₁ field map comprises determining a mean B₁ field strength representedin the B₁ field map and determining the mean flip angle from the mean B₁field strength.
 6. The method of claim 1, wherein acquiring the B₁ fieldmap comprises acquiring the B₁ field map during a pre-scan performedprior to the MRI scan, and wherein the B₁ field map represents astrength of a B₁ field at each pixel location of an imaging plane, theimaging plane located in a center of a bore of the MRI system.
 7. Themethod of claim 1, further comprising applying a mask to the B₁ fieldmap to generate a masked B₁ field map, and determining the mean flipangle from the masked B₁ field map.
 8. The method of claim 1, whereindetermining the transmit attenuation correction value based on the ratioof the mean flip angle to the prescribed flip angle comprisesdetermining the transmit attenuation correction value based on alogarithm of the ratio of the mean flip angle to the prescribed flipangle.
 9. A magnetic resonance imaging (MRI) system, comprising: atransmit radio frequency (RF) coil; an amplifier configured to drive thetransmit RF coil; and a controller coupled to the transmit RF coil andthe amplifier, the controller configured to: set a reference value oftransmit attenuation for the amplifier based on one or more anatomicalfeatures of an imaging subject to be scanned; acquire a two-dimensionalB₁ field map with the transmit attenuation set at the reference value;determine a mean flip angle from the B₁ field map; determine a transmitattenuation correction value based on a ratio of the mean flip angle toa prescribed flip angle; correct the reference value of transmitattenuation with the transmit attenuation correction value to obtain afinal value of transmit attenuation; and perform an imaging scan withthe transmit attenuation set at the final value.
 10. The system of claim9, wherein the two-dimensional B₁ field map is acquired by acquiring atwo-dimensional map of magnetic resonance (MR) signal phase shift duringa pre-scan, and deriving the two-dimensional B₁ field map by convertingthe MR signal phase shift into corresponding B₁ field strength.
 11. Thesystem of claim 10, wherein the map of MR signal phase shift is acquiredby using Bloch-Siegert shift.
 12. The system of claim 9, whereindetermining the mean flip angle from the B₁ field map comprisesdetermining a mean B₁ field strength represented in the B₁ field map anddetermining the mean flip angle from the mean B₁ field strength.
 13. Thesystem of claim 9, wherein the B₁ field map represents a strength of aB₁ field at each pixel location of an imaging plane, the imaging planelocated in a center of a bore of the MRI system.
 14. The system of claim9, wherein the controller is further configured to apply a mask to theB₁ field map to generate a masked B₁ field map, and determine the meanflip angle from the masked B₁ field map.
 15. A non-transitorycomputer-readable medium comprising instructions that, when executed,cause a processor to: set a reference value of transmit attenuation foran amplifier of a transmit radio frequency (RF) coil of a magneticresonance imaging (MRI) device; operate the MRI device to acquire atwo-dimensional B₁ field map with the transmit attenuation set at thereference value; determine a mean flip angle from the B₁ field map byaveraging values from the B₁ field map; determine a transmit attenuationcorrection value based on a ratio of the mean flip angle to a prescribedflip angle; correct the reference value of transmit attenuation with thetransmit attenuation correction value to obtain a final value oftransmit attenuation; and operate the MRI device to perform an imagingscan with the transmit attenuation set at the final value.
 16. Thecomputer-readable medium of claim 15, wherein to acquire thetwo-dimensional B₁ field map, the instructions, when executed, cause theprocessor to operate the MRI device to: acquire a two-dimensional map ofmagnetic resonance (MR) signal phase shift during a pre-scan; and derivethe two-dimensional B₁ field map by converting the MR signal phase shiftinto corresponding B₁ field strength.
 17. The computer-readable mediumof claim 16, wherein the map of MR signal phase shift is acquired byusing Bloch-Siegert shift.
 18. The computer-readable medium of claim 15,wherein the reference value of transmit attenuation is set based on oneor more anatomical features of an imaging subject to be scanned.
 19. Thecomputer-readable medium of claim 15, wherein determining the mean flipangle from the B₁ field map comprises determining a mean B₁ fieldstrength represented in the B₁ field map and determining the mean flipangle from the mean B₁ field strength.
 20. The computer-readable mediumof claim 15, wherein the B₁ field map is acquired during a pre-scanperformed prior to the imaging scan, and wherein the B₁ field maprepresents a strength of a B₁ field at each pixel location of an imagingplane, the imaging plane located in a center of a bore of the MRIdevice.